32 



CETACEA, 



a comparison of the tympanies with a large series of those of Mecja- 

 ptera hoops has convinced the writer that the fossil forms should be 

 included in the latter genus \ 



The tympanic is more inflated than in Balcenopfera, and has the 

 involucrum more distinctly pyriform ; the eustachian part of the 

 aperture is well-defined, and there are two well-marked ridges on 

 the inferior surface of the adult bone, of which the outermost is the 

 more prominent; the antero-internal extremity is usually more or 

 less pointed, the inner surface is convex (the convexity increasing 

 with age) and frequently has a vertical groove near the eustachian 

 channel, and the outer surface of the involucrum is very convex. 

 The cranium of the recent species is of moderate size, and the 

 yertebrse are intermediate in length between those of Baltpna 

 and Balcenoptera. 



Megaptera affinis, Van Beneden ". 



Syn. Megapteropsis rohusta, Van Beneden 3. 



This species is of large size and closely allied to M. hoops, but is 

 distinguished by the bluntness of the anterior extremity of the 

 tympanic and the greater flatness of its inner surface. 



Ilah. Europe (Belgium and England). 



M. 3531. Cast of an imperfect right tympanic, probably belonging 

 to an immature individual of the present species. The 

 original was obtained from the Coralline Crag of Sud- 

 bourn, near Orford, Suffolk, and is preserved in the 

 Museum of Practical Geology, Jermyn Street. The speci- 

 men is rather smaller than the one figured by Van 

 Beneden in the 'Ann. Mus. E. Hist. Nat. Belg.' vol. vii. 

 pt. 3, pi. xliii. figs. 1, 2, but agrees closely in form, and 

 especially in the blunt anterior extremity and flattened 

 inner wall, in both of which respects it differs from the 

 tympanic of M. similis (Van Beneden). The small deve- 

 lopment of the inferior ridges indicates that the specimen 

 is not adult. The original has been described and figured 



^ The recent tympanic figured for comparison with the fossil by Van Beneden 

 in the 'Ann. Mus. E. Hist. Nat. ]3elg.' vol. vii. pt. 3, pi. Ixxxix. figs. 15, 16, 

 under the name of Balcsnoptera antarctica, really belongs to Megaptera hoopsr; 

 compare B. M. No. 2. 76. 16. 18. 



-'' Bull. Ac. E. Belg. s6r. 2, vol. 1. p. 13 (1880). 



^ Ibid. vol. xxxiv. p. 15 (1872). The specific name was considered liable to 

 lead to confusion, and was accordingly changed. 



